Systems and methods for switching to a back-up power supply

ABSTRACT

Systems and methods for switching to a back-up power supply are provided. One such system includes a threshold detector circuit; a first switching circuit for enabling access to a first power source, the first switching circuit comprising at least a first transistor; and a second switching circuit for enabling access to a second power source, the second switching circuit comprising at least a second transistor; wherein the threshold detector is configured to cause the second switching circuit to enable access to the second power supply responsive to a voltage provided by the first power supply dropping below a predetermined threshold.

FIELD OF THE INVENTION

This invention relates in general to providing a back-up power supply,and more specifically to systems and methods for switching to a back-uppower supply.

DESCRIPTION OF THE RELATED ART

Much of today's electronic equipment needs a constant power source. Whena power supply fails, the switch to a backup supply should beinstantaneous such that the load voltage does not dip below a setthreshold. Typically diodes are used in an “OR” configuration (i.e.either the main supply or the backup supply delivers power to the load).Many applications, however, have tight voltage tolerances, and the lossthrough a diode is too great. Therefore, there exists a need for systemsand methods for addressing these and/or other problems related toproviding a back-up power supply.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdrawings. The components in the drawings are not necessarily drawn toscale, emphasis instead being placed upon clearly illustrating theprinciples of the present invention. In the drawings, like referencenumerals designate corresponding parts throughout the several views.

FIG. 1 is a block diagram depicting a power-switching circuit accordingto an embodiment of the invention.

FIG. 2A is a block diagram depicting an example of the thresholddetector shown in FIG. 1.

FIG. 2B is a graph illustrating a non-limiting example of a transienthysteresis effect within the threshold detector shown in FIG. 2A.

FIG. 3 is a block diagram depicting an example of the inverter shown inFIG. 1.

FIG. 4 is a block diagram depicting an example of an inverting switchshown in FIG. 1.

FIG. 5 is a block diagram depicting an example of a power switch shownin FIG. 1.

FIG. 6 is a block diagram depicting an example of a voltage supplycircuit.

FIG. 7 is a flow chart depicting a method according to one embodiment ofthe invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An embodiment of the invention enables sustained power to DC-inputend-use electronics. The embodiment is useful in applications that haveprimary and backup power sources. If the primary source fails, then thebackup source is supplied to the load instead of the primary source.Switches used in this embodiment are very low loss and can pass highcurrents to the load with very little drop in voltage.

A condition for switching between one source and another is a voltagelevel of the primary source (Vp). If Vp falls below a threshold set by acomparison circuit, then the load is powered by the back-up powersupply. Conversely, if Vp rises above the threshold, then the load ispowered by the primary power supply.

Low resistance field effect transistors (FETs) may be used as switches,and may be controlled by a threshold detection circuit. Using FETsenables a commercial “off the shelf” power source to be used, withoutthe need to have a higher voltage source to overcome diode losses.

Below is a detailed description of the accompanying 6 figures, whichillustrate a preferred embodiment of the present invention: FIG. 1depicts an embodiment of a power-switching circuit; FIGS. 2-5 depictexamples of components of the power-switching circuit; and FIG. 6depicts an example of a voltage supply circuit. Note, however, that theinvention may be embodied in many different forms and should not beconstrued as limited to the embodiments set forth herein. Furthermore,all examples given herein are intended to be non-limiting, and areprovided in order to help clarify the description of the invention.

FIG. 1 is a block diagram depicting a power-switching circuit 100according to an embodiment of the invention. The power-switching circuit100 may be used in many electronic devices that require a constant DCpower source. As a non-limiting example, among others, thepower-switching circuit 100 may be used in an up-converter deviceconfigured to increase the frequency of a signal.

As shown in FIG. 1, the power-switching circuit 100 includes a thresholddetector 102 that is coupled to voltages V_(B) and V_(P). The thresholddetector 102 compares the voltage V_(B) and the voltage V_(P) and isoperative to turn on or off an inverting switch 106-1 and an invertingswitch 106-2 responsive to whether the voltage V_(B) and the voltageV_(P) are within a predetermined value. The inverting switch 106-1 andthe inverting switch 106-2 are configured to turn on and off in acomplementary manner. In other words, when the inverting switch 106-1 isturned on, the inverting switch 106-2 is turned off and vice-versa.

The inverter 104 enables the inverting switch 106-1 to act in acomplementary manner to the inverting switch 106-2. In an alternativeembodiment the inverter 104 may be coupled between the thresholddetector 102 and the inverting switch 106-2. In a preferred embodiment,the inverting switch 106-1 and the inverting switch 106-2 are turned offat a time period set by R12 and C2 of FIG. 4, after a correspondingchange in the output of the threshold detector 102. Such a time periodmay vary between a few microseconds to over 100 milliseconds, dependingon the values of R12 and C2. In one embodiment, among others, the timeperiod may be 30 milliseconds. This delayed switching is implemented inorder to maintain a constant voltage output of the power-switchingcircuit 100.

The inverting switch 106-1 and the inverting switch 106-2 are coupled toa back-up power switch 108-1 and to a primary power switch 108-2,respectively. The back-up power switch 108-1 and the primary powerswitch 108-2 may be coupled to the voltage V_(B) and the voltage V_(P),respectively.

When 100 is in operation, the voltage V_(O) is substantially equal tothe voltage V_(P) if the voltage V_(P) is within a certain threshold,otherwise the voltage V_(O) is equal to the voltage V_(B). In thismanner, when a primary power source fails, a backup power source may beprovided to a load.

FIG. 2A is a block diagram depicting an embodiment of the thresholddetector 102 shown in FIG. 1. The threshold detector 102 receivesprimary voltage V_(P) and back-up voltage V_(B) as inputs and providesvoltage V₃ as an output. The threshold detector 102 includes acomparator A₁ which receives inputs via the connections 201 and 202, andprovides an output via a connection 206. The connection 201 is coupledto nodes 203 and 204. A resistor R₂ is coupled between node 203 andground, while a resistor R₁ is coupled between node 203 and back-upvoltage V_(B). The resistors R₁ and R₂ are configured to provide theconnection 201 with a predetermined fraction of the back-up voltageV_(B).

A resistor R₃ is coupled between the connection 202 and the primaryvoltage Vp. A resistor R₄ is coupled in series with capacitor C₁ betweenthe nodes 204 and 205. A resistor R₅ is coupled between the node 204 andthe node 205 (i.e., in parallel with R4 and the capacitor C₁). The node205 is coupled to the connection 206. A resistor R₆ is coupled betweenthe connection 206 and the supply voltage V_(S).

When the threshold detector 102 is in operation, the voltage V₃ is “low”if the primary voltage V_(P) is greater than a predetermined fraction ofthe back-up voltage V_(B). Conversely, when the primary voltage V_(P) isless than the predetermined fraction of the back-up voltage V_(B), thenthe voltage V₃ is “high.” This predetermined fraction is based on therelative values of the resistors R₁ and R₂ as well as the feedbacknetwork comprising the resistors R₄ and R₅, and the capacitor C₁.Preferably, the resistor R₅ establishes the steady-state component of“hysteresis” while resistor R₄ and capacitor C₁ create a transient“hysteresis” effect.

FIG. 2B is a graph 210 illustrating a non-limiting example of thetransient hysteresis effect created by the resistor R₄ and the capacitorC₁. Also illustrated are the settled values of the threshold created byresistor R₅. The settled values are given as levels 215 and 216. Thegraph 210 has a time axis 212 and a voltage axis 211. As shown in thisexample, when the primary voltage VP increases from 0V to its steadystate output level 220, the threshold 214 is lowered from level 215 tolevel 216 after transition period t₁. Conversely, as primary voltageV_(P) decreases from steady state output level 220 to 0V, the threshold214 is increased from level 216 to level 215 after transition period t₂(where t₂ is equal to t₁). This transient hysteresis (having transitionperiods t₁ and t₂) protects against rapid switching between powersources. Such rapid switching may occur when the source load changesfrom 0% to full load.

FIG. 3 is a block diagram depicting an embodiment of the inverter 104shown in FIG. 1. The inverter 104 receives voltage V₃ and outputsvoltage V₄. The inverter 104 includes a comparator A₂, which receivesinputs via connections 301 and 302, and provides an output viaconnection 303. A resistor R₇ is coupled between connection 301 andground, while a resistor R₈ is coupled from connection 301 to Vs. Thisdivides the voltage Vs to a lower value based on the values of resistorsR₇ and R₈. The connection 302 is coupled to the voltage V₃. A resistorR₉ is used to pull up the voltage at connection 303 to approximately Vswhen the voltage at 301 is greater than the voltage at 302. When theinverter 104 is in operation, the voltage V₄ is “low” when the voltageV₃ is “high” and vice versa.

FIG. 4 is a block diagram depicting an embodiment of an inverting switch106 (e.g., the inverting switch 106-1 or the inverting switch 106-2)shown in FIG. 1. The inverting switch 106 is coupled to voltage V₃ orvoltage V₄ at connection 401, and outputs voltage V₅ at the connection402.

The inverting switch 106 includes the transistors Q₁ and Q₂, which arecoupled as follows: the emitter of the transistor Q₁ is coupled to thecollector of the transistor Q₂; the collector of the transistor Q₁ iscoupled to the connection 402; a resistor R₁₀ is coupled between thebase of the transistor Q₁ and the connection 401; a resistor R₁₂ iscoupled between the base of the transistor Q₂ and the connection 401;the emitter of the transistor Q₂ is coupled to ground; a capacitor C₂ iscoupled between the base of the transistor Q₂ and ground; a resistor R₁₁is coupled between the collector of the transistor Q₁ and the supplyvoltage Vs. The transistors Q₁ and Q₂ may be, for example, bipolar npntransistors, among others.

When the inverting switch 106 is in operation, the value of the voltageat the connection 401 determines whether the transistors Q₁ and Q₂ areon (i.e., conducting between their respective collectors and emitters).The transistors Q₁ and Q₂ are turned on when the voltage at theconnection 401 is “high”, and vice versa. When the transistors Q₁ and Q₂are on, the voltage V₅ is “low,” and vice versa. The capacitor C2 causesa small delay (for example, among others, 30 milliseconds) between thetime that the voltage at the connection 401 transitions from “low” to“high” and the time that the transistor Q₂ turns on. A “high” to “low”transition at connection 401 immediately turns off transistor Q₁ whichcauses the voltage V₅ to transition “high” regardless of the turn offdelay of transistor Q₂. This “Instant on—delayed off” switching allowsfor a more constant voltage output of the power-switching circuit 100 bycompletely draining the old supply while the new supply is being loaded.

Resistor and capacitor values that may be used in the circuits depictedin FIGS. 2-4 may be, for example, among others, as follows: TABLE 1non-limiting examples of component values R₁  11 kilo-ohms R₂ 200kilo-ohms R₃  1 kilo-ohm R₄  51 kilo-ohms R₅ 510 kilo-ohms R₆  4.7kilo-ohms R₇  15 kilo-ohms R₈  15 kilo-ohms R₉  4.7 kilo-ohms R₁₀ 300kilo-ohms R₁₁  4.7 kilo-ohms R₁₂ 300 kilo-ohms C₁  0.1 MF (microfarads)C₂  0.1 MFNote that many alternative values for the resistors and capacitorsreferenced in Table 1 may be used, depending on a desiredimplementation.

FIG. 5 is a block diagram depicting an embodiment of a power switch 108(e.g., the back-up power switch 108-1 or the primary power switch 108-2)shown in FIG. 1. The power switch 108 is coupled to the voltage V_(P) orthe voltage V_(B) at a connection 502, and outputs the voltage V₆ at aconnection 503.

The power switch 108 includes transistors Q₃ and Q₄, which are coupledas follows: the gates of the transistors Q₃ and Q₄ are coupled to thevoltage V₅; the drains of the transistors Q₃ and Q₄ are coupled to eachother; the source of the transistor Q₃ is coupled to the connection 502;the source of the transistor Q₄ is coupled to the connection 503.

The power switch 108 is coupled to a corresponding power switch (e.g.,the back-up power switch 108-1 (FIG. 1) is coupled to the primary powerswitch 108-2). When the power switch 108 is in operation, the voltage V₅controls whether the voltage at the connection 502 is equal to thevoltage V₆ (the voltage at the connection 503). When the voltage V₅ ishigh, the transistors Q₃ and Q₄ are turned on, and the voltage V₆becomes equal to the voltage at the connection 502. Conversely, when thevoltage V₅ is low, the transistors Q₃ and Q₄ are turned off, and thevoltage V₆ becomes equal to the voltage provided at the connection 504by the corresponding power switch.

FIG. 6 is a block diagram depicting an embodiment of a voltage supplycircuit 600. The voltage supply circuit 600 includes diodes D₁ and D₂.The diode D₁ is coupled between connections 601 and 603, whereas thediode D₂ is coupled between connections 602 and 603. The voltages V_(P)and V_(B) are provided as inputs to the voltage supply circuit 600 atthe connections 601 and 602, respectively. The voltage supply circuit600 outputs the voltage Vs at the connection 603. The voltage Vs isequal to the voltage V_(B) or the voltage V_(P), whichever is higher.Examples of voltage supplies that may be used to provide the voltageV_(B) or the voltage V_(P) include, for example, among others, abattery, an AC to DC converter, or a DC/DC converter.

FIG. 7 is a flow chart depicting a method 700 according to oneembodiment of the invention. In step 701, a primary voltage is providedto a load. Then, a drop in the primary voltage below a predeterminedthreshold is detected, as indicated in step 702. The drop in primaryvoltage may, for example, be detected using a circuit that is configuredin the same or similar manner as the threshold detector 102 (FIG. 2A).

Responsive to the drop in the primary voltage, a first circuit having atleast one transistor (e.g., connected in-line) is used to provide aback-up voltage to the load, as indicated in step 703. In addition, asecond circuit having at least one transistor is used to disconnect theprimary voltage from the load, as indicated in step 704. The first andthe second circuits used for implementing steps 703 and 704,respectively, may, for example, each be configured in the same orsimilar manner as the power switch 108 shown in FIG. 5.

In an alternative implementation, the steps depicted in FIG. 7 may beexecuted out of order from that shown or discussed, includingsubstantially concurrently or in reverse order, as would be understoodby those of ordinary skill in the art. For example, steps 703 and 704may be executed substantially concurrently. Furthermore, the scope ofthe invention includes methods having fewer or additional steps thanshown in FIG. 7.

It should be emphasized that the above-described embodiments of thepresent invention are merely possible examples, among others, of theimplementations, setting forth a clear understanding of the principlesof the invention. Many variations and modifications may be made to theabove-described embodiments of the invention without departingsubstantially from the principles of the invention. All suchmodifications and variations are intended to be included herein withinthe scope of the disclosure and present invention and protected by thefollowing claims. In addition, the scope of the present inventionincludes embodying the functionality of the preferred embodiments of thepresent invention in logic embodied in hardware and/orsoftware-configured mediums.

1. A system comprising: a threshold detector circuit; a first switchingcircuit for enabling access to a back-up power source, the firstswitching circuit comprising at least a first transistor; and whereinthe threshold detector is configured to cause the first switchingcircuit to enable access to the back-up power source responsive to avoltage provided by a primary power source dropping below apredetermined threshold.
 2. The system of claim 1, further comprising: asecond switching circuit for enabling access to a primary power source,the second switching circuit comprising at least one transistor.
 3. Thesystem of claim 2, wherein the threshold detector is configured to causethe second switching circuit to enable access to the second power sourceresponsive to a voltage provided by a primary power source rising abovethe predetermined threshold.
 4. The system of claim 1, furthercomprising: an inverting switch coupled between the first switchingcircuit and the threshold detector.
 5. The system of claim 4, whereinthe inverting switch comprises at least one transistor.
 6. The system ofclaim 4, further comprising: an inverter coupled between the invertingswitch and the threshold detector.
 7. The system of claim 6, wherein theinverter comprises a comparator.
 8. The system of claim 1, wherein thefirst switching circuit comprises a second transistor coupled to thefirst transistor.
 9. The system of claim 8, wherein an emitter of thefirst transistor is coupled to a collector of the second transistor. 10.The system of claim 9, wherein current flow between respectivecollectors and emitters of the first and second transistors terminatesaccess to the back-up power source.
 11. The system of claim 9, whereinresistance to current flow between respective collectors and emitters ofthe first and second transistors enables access to the back-up powersource.
 12. A method comprising the steps of: detecting a primaryvoltage dropping below a predetermined threshold; enabling at least afirst transistor to provide access to a back-up voltage responsive todetecting the drop in the primary voltage.
 13. The method of claim 12,further comprising: enabling at least a second transistor to terminateaccess to the primary voltage responsive to detecting the drop in theprimary voltage.
 14. The method of claim 12, further comprising:detecting the primary voltage rising above the predetermined threshold;enabling at least a second transistor to provide access to the primaryvoltage responsive to detecting the rise in the primary voltage.
 15. Themethod of claim 14, further comprising: enabling the at least firsttransistor to terminate access to the back-up voltage responsive todetecting the rise in the primary voltage.
 16. A system comprising:means for detecting a primary voltage dropping below a predeterminedthreshold; means for enabling at least a first transistor to provideaccess to a back-up voltage responsive to detecting the drop in theprimary voltage.
 17. The system of claim 16, further comprising: meansfor enabling at least a second transistor to terminate access to theprimary voltage responsive to detecting the drop in the primary voltage.18. A system comprising: a threshold detector circuit; a first switchingcircuit for enabling access to a back-up power source, the firstswitching circuit comprising at least a first transistor; a secondswitching circuit for enabling access to a primary power source, thesecond switching circuit comprising at least one transistor; aninverting switch coupled between the first switching circuit and thethreshold detector; and an inverter coupled between the inverting switchand the threshold detector; wherein the threshold detector is configuredto cause the first switching circuit to enable access to the back-uppower source responsive to a voltage provided by a primary power sourcedropping below a predetermined threshold; and wherein the thresholddetector is configured to cause the second switching circuit to enableaccess to the second power source responsive to a voltage provided by aprimary power source rising above the predetermined threshold.
 19. Athreshold detection circuit for enabling access to a back-up powersource, comprising: a comparator having a first input connection forreceiving a first input voltage, a second input connection for receivinga second input voltage, and an output connection for providing an outputvoltage; a first resistor and a first capacitor that are coupled inseries between the first input connection and the output connection; anda second resistor that is coupled between the first input connection andthe output connection, and that is coupled in parallel to the series offirst resistor and the first capacitor.
 20. The threshold detectioncircuit of claim 19, wherein the threshold detection circuit isconfigured to cause a first switching circuit to enable access to theback-up power source responsive to a voltage provided by a primary powersource dropping below a predetermined threshold.